Integrated gate assisted turn-off, amplifying gate thyristor, and a method for making the same

ABSTRACT

An integrated amplifying gate thyristor is provided with an integral diode in the thyristor structure in the same semiconductor body. The diode provides gate assist turn-off capability with the same gate electrode used to turn-on a pilot thyristor of the device. The common cathode-base region of the pilot and main thyristors also is common with the anode region of the diode. The current gain of the NPN transistor structure formed at the diode and the common anode-base region at the diode is less than the ratio of I FB  /I g , where I FB  is the forward anode current on triggering the main thyristor into the low impedance conduction state by applying a threshold negative gate assist current (I G ) and an operating anode-cathode load potential, and I g  is a negative gate current selected to assist in turn-off of the main thyristor. Preferably, the current gain of said NPN transistor structure at the diode is controlled by selectively irradiating the diode preferably with electron radiation, by adding a shallow impurity lip to the cathode region of the diode toward the main thyristor, or both.

This is a division of application Serial No. 581,255 filed May 27, 1975.

FIELD OF THE INVENTION

The present invention relates to semiconductor devices and particularlyamplifying gate thyristors having gate assisted turn-off capability.

BACKGROUND OF THE INVENTION

Thyristors are nonlinear, solid state devices that are bistable. Thatis, they have both a high impedance state and a low impedance state. Forthis reason, thyristors are generally used as solid state switches.Thyristors commonly have four-layer PNPN semiconductor structures, withtwo intermediate regions called cathode-base and anode-base regions, andtwo extremity regions adjoining major surfaces of a semiconductor bodycalled cathode-emitter and anode-emitter regions. Thyristors are usuallygated or switched from a high impedance blocking state to a lowimpedance conducting state by applying through a gate electrode apositive electrical control signal to the cathode-base region of thedevice.

In operation, a positive control signal forward biases the PN junction(cathode junction) between the cathode-emitter and cathode-base regionsand causes electron injection into the cathode-base region in thevicinity of the reverse biased, forward blocking center PN junctionbetween the cathode-base and anode-base regions. The injected electronsare swept across the depletion layer at the forward blocking junction,causing an anode-to-cathode electric current and increasing the currentgain (α's) of the PNP and NPN transistor equivalents of the structure.If the positive control signal is high enough, the sum of the gains (αs)of the NPN and PNP structure equal unity in some portion or filament,and the device will switch the thyristor from the high impedance,blocking state to the low impedance, conducting state. The thyristorwill thereafter remain in the conducting state so long as the currentthrough the thyristor exceeds the holding current of the device.

A major restriction on power thyristors has been the dI/dt capability,i.e. the rate of current increase or "turn-on" as a function of time.The difficulty is that only a small portion of the device is responsiveto the control signal and initially switches to the conducting state.The device is dependent upon carrier diffusion to turn-on the remainderof the active regions, which require substantial time. Initially, onturn-on, the anode-to-cathode voltage drops instantaneously to about 10%of the blocking state value, and the current is conducted through theportions or filaments of the device in the conducting state, causing avery high current density and localized heating and degrading of thedevice. To avoid such degradation and possible failure of the thyristor,the external circuit typically requires an inductance to limit thecurrent rise on switching of the thyristor, which causes power lossesand generally limits the performance of the circuit.

Amplifying gate thyristors, such as the one shown in FIG. 1, have beendeveloped to provide improved dI/dt capability in power thyristors andto reduce the current carrying requirements of the gate circuit. Anauxiliary or pilot thyristor 2 of annular shape is provided preferablycentrally of the main thyristor 1 in the same semiconductor body. Pilotthyristor 2 and main thyristor 1 have their anode-emitter, anode-baseand cathode-base regions 3, 4 and 5 in common, and the cathode-emitterregions 6 and 7 of the pilot and main thyristors are spaced adjacenteach other along the same major surface of the semiconductor body. Agate electrode 8 is provided adjacent and preferably centrally of thepilot thyristor opposite from the main thyristor, and a floatingelectrode 9 is provided on the major surface of the semiconductor bodyastride the PN junction between the cathode-emitter and cathode-baseregions of pilot thyristor 2.

Pilot thyristor 2 is turned-on by a control signal applied to gateelectrode 8 which flows laterally into pilot thyristor 2 as shown byarrows 10, forward biasing the cathode junction between thecathode-emitter and cathode-base regions 5 and 6, and turning-on thepilot thyristor 2 from the inner edge of the cathode-emitter where thegate current is injected. The resulting anode current as shown by arrows11 through the pilot thyristor 2 is utilized as a gate current toturn-on main thyristor 1. The anode current from pilot thyristor 2 flowsthrough floating contact 9 and cathode-base region 5 to the cathodeshunts 12 along the inner edge of cathode-emitter region 7 of mainthyristor 1 as shown by arrows 13. A substantial portion of the mainthyristor, particularly if the structure is interdigitated, can thus beinitially switched to the conducting state. Such amplifying thyristorscan be utilized to rapidly switch high power without substantial powerlosses. Cathode shunts 12 also provide increased dv/dt capability byconducting anode current without forward biasing the cathode junctionand producing a lateral current flow that more rapidly forward biasesthe cathode junction.

Thyristors are also notorious for their long turn-off times. That is,the time required to establish the high impedance, blocking state in thethyristor on switching from the low impedance, conducting state. In asimple thyristor structure, the blocking state can be restored only byreducing the anode-to-cathode current to below the holding current forsuch a time period to allow the depletion layer to be reformed at theforward blocking center junction when forward voltage is reapplied. Theturn-off time is thus directly related to the diffusion time of thecarriers, both electrons and holes, across the base regions and to thecarrier lifetimes within the base regions.

Where rapid turn-off capability has been needed, an interdigitated gateelectrode structure has been provided with the cathode-emitter regionand cathode electrode. A negative control signal is applied to theinterdigitated gate electrode(s) to cause a current flow from thecathode electrode and reverse bias the cathode junction between thecathode-base and cathode-emitter regions. The current density in thedevice can thus be counteracted after rapid reduction of the loadcurrent to zero to avoid refiring of the device by rapid reapplicationof the load potential, e.g. in high frequency operation of 10 to 20 KHz.Or, the high impedance blocking state can thus be reestablished in thethyristor while a load current is still applied to the device under lowfrequency or DC load potentials. Thyristors operated in the former modeare commonly called "Gate Assisted Turn-Off Thyristors" or "GATTS;" andthyristors operated in the latter mode are commonly called "GateControlled Switches" or "GCSs."

Gate assisted turn-offs cannot be effectively performed in thyristorswith amplifying gates for turn-on unless separate gates are utilized forturn-on and turn-off. Presently known amplifying gate designs have ahigh lateral resistance (e.g. 10-25 ohms) in the direction in whichcurrent must flow during turn-off. The current for turn-off is ofopposite polarity from the gate current for turn-on, and the cathodejunction between the cathode-emitter and cathode-base of the pilotthyristor is reverse biased. The current must, therefore, flow laterallythrough cathode-base region 5 under cathode-emitter region 6 to reachgate electrode 3. The resulting high resistance will, for a given gatevoltage, greatly reduce the gate turn-off current and in turn reduce theeffectiveness of the gate assist turn-off. In addition, degradation ofthe device may occur due to nonuniform avalanche of the cathode junctionbetween the cathode-emitter and cathode-base regions of the pilotthyristor 2. For this reason, thyristor designers have avoided the useof amplifying gate devices where a gate assisted turn-off is required.

Gate assisted turn-off capability has been provided in amplifying gatethyristors by providing a separate turn-off gate through which the anodecurrent from the main thyristor may be shunted without passing throughthe pilot thyristor. A diode 14 is provided to block current flowthrough said separate gate during turn-on. Diode 14 is typically fusedto floating electrode as shown in FIG. 1, which doubles as the turn-offgate.

These separate, rectified turn-off gates are difficult and expensive tofabricate and package. Moreover, such rectified turn-off gate has notbeen heretofore successfully provided integral with the thyristorstructure in the same semiconductor body. The difficulty is that theforward biasing of the PN junction of the diode causes an anode current,impairing the effectiveness of the gate assist turn-off.

The present invention overcomes these difficulties of prior devices. Anamplifying gate thyristor is provided with an integral diode in thethyristor structure in the same semiconductor body. Moreover, theamplifying gate thyristor is capable of performing gate assistedturn-offs through the same gate electrode used to turn-on the pilotthyristor.

SUMMARY OF THE INVENTION

An integrated amplifying gate thyristor with gate assist turn-offcapability is comprised of main and pilot thyristors disposed in asemiconductor body having first and second major surfaces. Eachthyristor has four impurity regions, three of which are common to theother thyristor, extending through the body between the major surfaces.The impurity regions of each thyristor are of alternateconductivity-type disposed alternately through the body, with PNjunctions formed between adjacent impurity regions. The two impurityregions adjoining the first and second major surfaces arecathode-emitter and anode-emitter regions, respectively; and the twoimpurity regions interior of the body adjoining the cathode-emitter andanode-emitter regions are cathode-base and anode-base regions.

The cathode-base, anode-base and anode-emitter regions are common toboth thyristors. The common cathode-base region adjoins the first majorsurface of the semiconductor body at least between the thyristors; andthe cathode-emitter regions of the main and pilot thyristors areadjacent to and spaced apart from each other along the first majorsurface. Preferably, the pilot thyristor is positioned centrally of themain thyristor, the cathode-emitter region of the pilot thyristor isformed in annular segments, and the main thyristor provided with cathodeshunts.

A diode to provide for gate assist turn-off capability is disposed inthe semiconductor body integrally with the thyristor structure. Thediode has anode and cathode impurity regions, with the anode regioncommon with the cathode-base region of the thyristors. The cathoderegion is a separate impurity region disposed in the semiconductor bodyadjoining the first major surface adjacent to and spaced from thecathode-emitter regions of both the main and pilot thyristors, and is ofan impurity of a conductivity type to form a separate PN junction withthe common cathode-base region of the thyristors.

The current gain of the transistor structure formed by the diode and thecommon anode-base region at the diode is less than the ratio of I_(FB)/I_(g), where I_(FB) is the forward anode current on triggering the mainthyristor into the low impedance conduction state by applying athreshold negative gate assist current (I_(G)) and an operatinganode-cathode load potential, and I_(g) is a selected gate assistturn-off current to turn-off the main thyristor. Said current gain ispreferably established by localized control of the carrier lifetime inthe common cathode-base region at the diode, which may be done in anysuitable way. The width and impurity concentration of the commoncathode-base region at the diode may be locally varied to lower thetransport factor for electrons through that region. The impurityconcentration of the cathode region of the diode and the commoncathode-base region at the diode can be controlled to reduce theinjection efficiency for electrons into the cathode-base region at thediode.

Preferably, however, the current gain is controlled by decreasing thecarrier lifetime in the common cathode-base region at the diode. Thatmay be accomplished by selective diffusion of gold into the commoncathode-base region at the diode. Preferably, the carrier recombinationrate is increased without correspondingly increasing the carrierregeneration rate by selectively irradiating the diode most desirably byelectron radiation, preferably of energy between 1 and 3 Mev, andpreferably to a dosage level greater than 1× 10¹³ electrons/cm².Additionally, a shallow impurity lip of preferably greater than the areaand less than one-half of the depth of the deep portion is added to thecathode region of the diode toward the cathode-emitter region of themain thyristor to reduce the current gain of the NPN structure at thediode at the part where injection occurs on the gate assisted turn-off.This shallow impurity lip also allows for increased depth of the deepportion of the cathode region (without a corresponding increase incurrent gain) to provide a high lateral resistance in the commoncathode-base region at the diode and avoid the waste of gate current onturn-on.

The thyristor is completed by positioning electrodes on the majorsurfaces of the semiconductor body. Cathode and anode electrodes arepositioned on said first and second major surfaces, respectively, tomake preferably low resistance, ohmic contact with the cathode-emitterregion of the main thyristor and with the common anode-emitter region ofthe main and pilot thyristors. A gate electrode is positioned on thefirst major surface to make preferably low resistance, ohmic contactwith the common cathode-base region adjacent to and spaced from thecathode-emitter region of the pilot thyristor, and with the cathoderegion of the diode. A floating electrode is positioned on the firstmajor surface adjacent to and spaced from the gate electrode to makepreferably low resistance, ohmic contact with the common cathode-baseregion between the thyristors and the cathode-emitter region of thepilot thyristor. Preferably, the floating electrode is alsointerdigitated with the cathode-emitter region of the main thyristorwithout making ohmic contact with said cathode-emitter region.

Preferably the pilot thyristor is positioned centrally of the mainthyristor. The cathode-emitter region of the pilot thyristor and thecathode region of the diode are preferably formed in spaced apartannular segments. The annular segments of the cathode-emitter region ofthe pilot thyristor may also be offset from the annular segments of thecathode region of the diode to facilitate alignment of an irradiationmask and in turn selective irradiation of the diode to reduce thecurrent gain of said NPN transistor structure at the diode.

Other details, objects and advantages of the invention will becomeapparent as the following description of the presently preferredembodiments and presently preferred methods of performing the sameproceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, the presently preferred embodiments of theinvention and presently preferred methods of performing the inventionare illustrated, in which:

FIG. 1 is a cross-sectional view in elevation of a prior art amplifyinggate thyristor with gate assist turn-off capability;

FIG. 2 is a partial top view, with portions broken away and a suitableirradiation mask shown in chain line, of integrated amplifying gatethyristor with gate assist turn-off capability of the present invention;

FIG. 3 is a cross-sectional view in elevation taken along lines III--IIIof FIG. 2;

FIG. 4 is a top view, with portions broken away and a suitableirradiation mask shown in chain line, of a second integrated amplifyinggate thyristor with gate assist turn-off capability of the presentinvention;

FIG. 5 is a cross-sectional view in elevation taken along lines V--V ofFIG. 4;

FIG. 6 is a top view, with portions broken away, of a third integratedamplifying gate thyristor with gate assist turn-off capability of thepresent invention;

FIG. 7 is a cross-sectional view in elevation taken along lines VII--VIIof FIG. 6; and

FIG. 8 is an enlarged fragmentary cross-sectional view in elevation of aportion of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a prior art amplifying gate thyristor with gateassist turn-off capability is shown for purposes of comparativeillustration. Discussion of this prior art device can be found in the"Background of the Invention".

Referring to FIGS. 2 and 3, semiconductor body 20 is provided forforming an integrated amplifying gate thyristor with gate assistturn-off capability of the present invention. Semiconductor body 20 istypically a commercially available single crystal silicon wafer ofcircular shape having a thickness typically of about 8 to 20 mils, andhaving first and second opposed major surfaces 21 and 22.

Provided in semiconductor body 20 are suitable impurities, typically ina pattern having circular symmetry, to form a main thyristor 23 of fourimpurity regions 24, 25, 26 and 27 of alternate type conductivitydisposed alternately through the body 20 from first major surface 21 tosecond major surface 22. PN junctions 28, 29 and 30 (cathode, forwardblocking and anode junctions, respectively), are thus formed betweenimpurity regions 24 and 25, 25 and 26, and 27 and 28, respectively.Impurity regions 24 and 27 adjoin first and second major surfaces 21 and22, respectively, to form cathode-emitter region 24 and anode-emitterregion 27 of main thyristor 23; and impurity regions 25 and 26 aredisposed in body 20 adjoining cathode-emitter and anode-emitter regions24 and 27 to form cathode-base region 25 and anode-base region 26 ofmain thyristor 23. Impurity region 25 also adjoins first major surface22 centrally and peripherally around cathode-emitter region 24, andpreferably intermittently cathode-emitter region 24 to form cathodeshunts 25A.

Impurities are also provided in semiconductor body 20 to form pilotthyristor 31 with cathode-base, anode-base and anode-emitter regionscommon to main thyristor 23. At least one additional impurity region 32is provided adjoining first major surface 21 adjacent to and spacedpreferably centrally from cathode-emitter region 24 of main thyristor23. Impurity region 32 is of a conductivity type to form PN junction 33with said common cathode-base region 25 and form separatecathode-emitter region 32 for pilot thyristor 31.

Also provided in semiconductor body 20 is an impurity to form diode 34having its anode region common with common cathode-base region 25 ofthyristors 23 and 31. Diode 34 has its cathode region formed by animpurity region 35 adjacent to and spaced from cathode-emitter regions24 and 32 of main and pilot thyristors 23 and 31. Impurity region 35 isof a conductivity type to form PN junction 36 with common cathode-baseregion 25 of the thyristors.

Preferably, pilot thyristor 31 is positioned centrally of main thyristor23, as shown in FIGS. 2 and 3, to provide for most efficient utilizationof the anode current through pilot thyristor 31 to turn-on mainthyristor 23. In addition, cathode-emitter region 32 of pilot thyristor31 and cathode region 35 of diode 34 are typically formed in spacedapart annular segments, for example, as illustrated in FIGS. 2 and 3, tomost easily fabricate the structure.

Typically, the impurities and impurity regions as above described areprovided in the semiconductor body 20 by standard diffusion techniques.Alternatively, the impurity regions may be provided by epitaxial growthor a combination of epitaxial growth and diffusion.

To illustrate, the impurities and impurity regions are typicallyprovided by commercially obtaining semiconductor body 20 uniformly dopedwith an N-type impurity, such as phosphorus or arsenic, to aconcentration typically between about 5 × 10¹³ and 5 × 10¹⁴ atoms/cm²,i.e. 200 to 10 ohm-cm. Body 20 is typically diffusion doped with aP-type impurity such as boron, gallium or aluminum through first andsecond major surfaces 21 and 22 by standard diffusion techniques to formcathode-base and anode-emitter regions 25 and 27. Anode-base region 26is formed at the same time between regions 25 and 27 by the residualN-type impurity of the body. Anode-base region 26 typically has athickness between about 50 and 250 microns, depending upon desiredvoltage rating of the device. Cathode-base and anode-emitter regions 25and 27 have surface impurity concentrations of typically between about 5× 10¹⁵ and 1 × 10¹⁸ atoms/cm³. If desired, to reduce the resistance ofanode-emitter region 27 and lower the forward voltage drop of the devicein the conduction mode, first major surface 21 may be masked and thediffusion continued to raise the impurity concentration of anode-emitterregion 27 adjacent second major surface 22 to at least 1 × 10¹⁸atoms/cm³.

After the initial diffusion, first and second major surfaces 21 and 22are masked with a standard diffusion mask such as silicon dioxide.Typically this masking is accomplished by heating semiconductor body 20in an oxygen rich atmosphere such as steam to about 1200°-1250° C. for 3to 4 hours. Window patterns suitable for simultaneously formingcathode-emitter regions 24 and 32 of main and pilot thyristors 23 and31, respectively, and cathode-region 35 of diode 34 are then opened inthe masking layer covering first major surface 21 by standardphotolithographic and etch techniques. Impurity regions 24, 32, and 35are then simultaneously provided into semiconductor body 20 through theopened window pattern and exposed portions of first major surface 21 bydiffusion of an N-type impurity such as phosphorus by a standarddiffusion method. Cathode-emitter regions 24 and 32 and cathode region35 are thus formed adjoining first major surface 21 adjacent to andspaced apart from each other with a surface concentration typically ofabout 1 × 10¹⁹ to 1 × 10²¹ atoms/cm³ and a diffusion depth typically ofabout 10 to 20 microns.

The current gain of the NPN transistor structure formed by cathoderegion 35 of diode 34, and common cathode-base and anode-base regions 25and 26 at diode 34 is provided less than the ratio of I_(FB) /I_(g),where I_(FB) is the forward anode current on triggering of the mainthyristor into the low impedance conducting state by applying athreshold negative gate assist current (I_(G)) and an operatinganode-cathode load potential, and I_(g) is a selected gate assistturn-off current to turn-off main thyristor 23. The current gain (α) iscalculated by dividing the forward anode current (I_(FB)) by thethreshold negative gate assist current (I_(G)) measured in the same way.The "threshold" negative gate assist current is the minimum negativegate assist current to trigger the main thyristor under the operatinganode-to-cathode load potential. This "operating" load potential will,of course, vary with whether the thyristor is operated in the GATT orGCS mode. In either case, the operating load potential is at the highestvoltage and highest rate of change of voltage at which the thyristor isdesigned to operate. All determinations are also made at the highesttemperature at which the thyristor is designed to operate. In thisconnection, it should be noted that some anode current is associatedwith the function of the diode when the thyristor is in a high impedanceblocking state.

Maintaining the current gain of the NPN structure below said ratioenables the main thyristor to be turned-off by gate assist throughforward bias of PN junction 36 of diode 34 without causing the mainthyristor to retrigger into the conduction mode. The negative gateassist bias can thus be efficiently utilized to assist in the turn-offof the main thyristor as hereinafter more fully described. To understandthe reason for maintaining said current gain below said ratio, considerwhat would happen if the current gain were not below said ratio: whenthe negative gate assist is applied, the current is drawn out of thedevice through the gate electrode and electrons are injected into commoncathode-base region 25 through junction 36. Many of these electronsreach forward blocking junction 29 and flow through into anode-baseregion 26. These electrons are base current to the PNP transistorstructure formed by the cathode-base, anode-base and anode-emitterregions 25, 26 and 27 and cause collector-to-emitter current. Theimpedance of the circuit of main thyristor 23, which is typicallydesigned to conduct several hundred amperes, is lower than the gatecircuit, which is typically designed to carry a few amperes.Accordingly, the anode-to-cathode load potential drives saidcollector-to-emitter current of the PNP transistor structure primarilyinto the common cathode-base region 25 at the main thyristor, forwardbiasing cathode junction 28 and triggering main thyristor 21. The mainthyristor is prevented from triggering in this manner in the presentinvention by maintaining the current gain of the NPN transistorstructure at the diode low and minimizing thegate-assist-current-induced electrons reaching forward blocking junction29.

The current gain of the transistor structure formed by cathode region 35of diode 34 and common cathode-base and anode-base regions 25 and 26 atdiode 31 is controlled by tailoring the geometry and injectionefficiency of cathode region 35, the width and impurity distribution ofcommon cathode-base region 25 at diode 31, and the carrier recombinationrate in common cathode-base region 25 at diode 31. These parameters maybe controlled in any suitable way including separately diffusingimpurity region 35 to vary its depth and impurity concentration.Preferably, however, cathode region 35 is simultaneously diffused withimpurity regions 24 and 32, and the carrier recombination rate of commoncathode-base region 25 at diode 34 varied to provide the desiredlocalized current gain. Said carrier recombination rate may beestablished in any desired way such as by selective diffusion of gold toprovide recombination centers. Gold diffusion is, however, not preferredbecause of added masking and diffusion steps, the difficulty incontrolling the diffusion depth, and the leakage current which results.Most desirably, the carrier recombination rate in common cathode-baseregion 25 at diode 34 is tailored by selective irradiation of diode 34as more fully described hereinafter in connection with FIGS. 4 and 5.

Preferably, at this stage in the fabrication, portions 21A of firstmajor surface 11 between cathode-emitter region 32 and cathode region 35are selectively etched by standard photolithographic masking and etchingtechniques. This etch removes the highly conductive (diffusion) layer inthose areas. The current conductivity in those areas is reduced and theamount of wasted current in both the turn-on and turn-off modes isreduced by preventing gate current from flowing through commoncathode-base region 25 between the segments of cathode-emitter region 32of pilot thyristor 31 and cathode region 35 of diode 34, bypassing pilotthyristor 31 during turn-on and diode 34 during turn-off.

After completion of diffusions and preferred selective etch,semiconductor body 20 is disposed on anode electrode 37 with secondmajor surface 22 of the body in contact with the major surface of theelectrode. Electrode 37 is usually separately formed of molybdenum ortungsten in a circular shape as least as large as semiconductor body 20and is generally 50 to 80 mils thick. Electrode 37 is alloyed to secondmajor surface 22 of the semiconductor body by heating electrode 37 andbody 20 in intimate contact in an inert atmosphere such as argon to atemperature typically of about 680° C. In this way, electrode 37 makespreferably low resistance, ohmic contact with common anode-emitterregion 37 across the entire second major surface 22.

Cathode-electrode 38, gate electrode 39 and floating electrode 40 arethen preferably simultaneously formed on first major surface 21.Cathode-electrode 38 makes preferably low resistance, ohmic contact withcathode-emitter region 24 of main thyristor 23 and is preferably apatterned electrode in the form of a "snowflake" design to provide forgate interdigitation. Cathode-electrode 38 also makes ohmic contact withcommon cathode-base region 25 at cathode shunts 25A and at the peripheryto assist in providing high dv/dt capability. Gate electrode 39 istypically generally circular with radially extending portions 41 asshown in FIG. 2, to make preferably low resistance, ohmic contact withthe common cathode-base region 25 adjacent to and spaced fromcathode-emitter region 32 of pilot thyristor 31, and with cathode region35 of diode 34. Floating electrode 40 is typically an interdigitatedstructure with radially extending portions 42A interdigitated withcathode electrode 38, and makes low resistance, ohmic contact withcommon cathode-base region 25 between segments of main thyristor 23, andradially extending portions 42B to make preferably low resistance, ohmiccontact with common cathode-base region 25 between main and pilotthyristors 23 and 31 and cathode-emitter region 32 of pilot thyristor31.

Electrodes 38, 39 and 40 may be of any suitable metal, such as aluminum,which will provide an ohmic contact and preferably a low resistance,ohmic contact to the semiconductor body. Preferably, the material forthe electrodes is also one, such as aluminum, which can be readily vaporor sputter deposited on the semiconductor body. Preferably, electrodes38, 39 and 40 are vapor or sputter deposited to a thickness typically ofabout 50,000 to 100,000. A. Electrodes 38, 39 and 40 are selectivelyformed as above described and shown in FIG. 2 by deposition over theentire surface followed by selected removal of the deposited layer bystandard photolithographic masking and etching techniques. As formed,electrodes 38, 39 and 40 typically make low resistance ohmic contact asabove described; however, sintering to achieve the lower resistance ofthe contact may be appropriate in certain embodiments.

The integrated amplifying gate thyristor is typically completed bylapping side surfaces 43 by known procedures to bevel said side surfacesand shape the electric fields in the thyristor and in addition,spin-etching to reduce edge leakage and the possibility of localizedsurface breakdown during operation. Beveled side surfaces 43 are thencoated with a suitable passivating layer 44. Material particularlysuitable for this purpose is 1,2-dihydroxyanthraquinone (also called"alizarin") in combination with a silicone or epoxy resin. Passivatinglayer 44 substantially reduces atmospheric effects on the semiconductorbody.

Selective irradiation may thereafter be performed on diode 34 to reducethe current gain of the NPN transistor formed by cathode region 35 ofdiode 34 and common cathode-base and anode-base regions 25 and 26 atdiode 34. First, those portions of first major surface 21 ofsemiconductor body 20 adjoining main and pilot thyristors 23 and 31 areselectively masked against a radiation means, while leaving exposedthose portions of first major surface 21 adjoining diode 34. Theselective masking is typically accomplished with a circular shield plate50 larger than semiconductor body 20, having openings 51 thereincorresponding in size and shape to diode 34. Shield plate 50 ismechanically positioned over first major surface 21 of body 20 withopenings 51 corresponding to diode 34. Shield plate 50 is of sufficientdensity and thickness to be opaque to the particular radiation to beused. For electron radiation, shield plate 50 may be of standard, lowcarbon steel about 1/4 inch thick or tungsten or lead about 0.06 inchthick.

After placement of shield plate 50 to selectively mask, diode 34 isselectively irradiated with radiation 52 from a suitable radiationsource or means. Any kind of radiation may be appropriate provided it iscapable of bombarding and disrupting the atomic lattice of thesemiconductor body 20 to create energy levels that substantiallyincrease the carrier recombination rate and decrease carrier lifetimewithout significantly increasing the carrier generation rate. Electronirradiation is preferably used as a suitable irradiation means becauseof availability and inexpensiveness. It is contemplated, however, thatradiation such as proton, neutron and alpha radiation may be appropriateprovided it is of proper intensity and duration. It is further preferredthat the energy of electron irradiation be between 1 Mev. and 3 Mev.Lower energies are generally believed to result in substantially elasticcollisions with the atomic lattice and, therefore, do not provide enoughdislocations within the semiconductor lattice in a commercially feasibletime. Conversely, electron radiation above 3 Mev. in energy is believedto cause too severe a lattice damage to the silicon crystal to maintaincertain other electrical characteristics of the device within nominalvalues.

It is further preferred that the radiation dosage of electron radiationbe greater than 1 × 10¹³ electrons/cm² to sufficiently increase thecarrier recombination rate and, in turn, decrease the current gain inthe common cathode-base region 25 at diode 34. The particular radiationdosage will, of course, vary with the I_(FB), I_(g) and I_(G) of theparticular device. For the particular thyristor utilized inexperimentation with a gate assist current (I_(g)), a radiation dosagelevel with electron radiation of between 3 and 8 × 10¹⁴ electrons/cm²was found to be satisfactory. A current gain of 0.2 was therebyprovided, which has been found suitable for a 2 amp gate assistturn-off. On the same device, a 0.1 current gain is anticipated to beappropriate for a 4 amp gate assist turn-off, and 0.4 current gain for a1 amp gate assist turn-off.

By this selective radiation technique, the main and pilot thyristors areessentially non-irradiated with said radiation means because of theselective masking. "Non-irradiated" in this context may, however, simplymean irradiating to a lower level of irradiation where the electricalcharacteristics of the device are not adversely effected. Moreover, the"non-irradiated" portions of the devices means radiated to a lower levelfor this purpose. Other portions of the "non-irradiated" portions of thethyristor may and most likely will be selectively irradiatedconcurrently or sequentially to effect other electrical characteristicsof the device.

After irradiation is completed, shield plate 50 is physically removedfor reuse and subsequent irradiation.

The operation of the integrated amplifying gate thyristor proceeds byfirst applying an operating load voltage between anode electrode 37 andcathode electrode 38 across the semiconductor body 20. The applied loadvoltage is such that the main and pilot thyristors are in a forward highimpedance blocking state. A positive current suitable for turn-on of thepilot thyristor is then applied to gate electrode 39. Such gating signalcauses a lateral current flow as shown by arrow 45 to forward biascathode junction 33 of pilot thyristor 31. Forward biasing of cathodejunction 33 causes carrier injection into common cathode-base region 25at pilot thyristor 31, and in turn causes pilot thyristor 31 to switchto a low impedance, conducting state, with anode current shown by arrow46 flowing from anode electrode 37 into cathode-emitter region 32 ofpilot thyristor 31. The anode current in turn flows from thecathode-emitter region 32 into floating electrode 40, commoncathode-base region 25 and cathode shunts 25A located along the inneredge of cathode-emitter region 24 of main thyristor 23 to cathodeelectrode 38, as shown by arrows 47. Although pilot thyristor 31 isdisposed in body 20 in annular segments, floating electrode 40 includesan annulus so that the anode current from the pilot thyristor isinjected into main thyristor 23 along the entire inner edge ofcathode-emitter region 24. The injected anode current from pilotthyristor 31 forward biases the cathode junction 28 of main thyristor 23and causes carrier injection into common cathode-base region 25 andswitching of main thyristor 23 from the high impedance blocking state toa low impedance conducting state with the flow of anode current shown byarrows 48.

To turn-off main thyristor 23 operated in the GATT mode (e.g. under ahigh frequency AC load potential above 10 KHz), the anode current iscommutated to zero leaving a high carrier density in anode-base region26 of the thyristor adjacent PN junction 29. A negative gate assistcurrent is applied to gate electrode 39 as the forward lead voltage isreapplied to cause a lateral current to flow from cathode electrode 38through cathode shunts 25A, cathode-base region 25 and diode 34 to gateelectrode 39. Such lateral current tends to reverse bias to cathodejunction 28, and counteracts the current distribution present in thecathode-base region that tends to forward bias cathode junction 28. Thegate assist thereby reduces the likelihood that a reapplied forwardvoltage will fire the device, and the forward voltage can be reappliedwithin a shorter time period after commutation of the anode load currentto zero -- by definition reducing the "turn-off time" of the device.

To turn-off main thyristor 23 operated in the GCS mode (i.e. under a lowfrequency AC or DC load voltage), a negative gate current is applied togate electrode 39 while the load voltage remains applied between theanode and cathode electrodes 37 and 38. The negative gate signal forwardbiases PN junction 36 of diode 34, causing lateral current flow as shownby arrows 49 from cathode electrode 38 through cathode shunts 25A,cathode-base region 25 and diode 34 to gate electrode 39. The voltage onPN junction 28 of main thyristor 23 is reduced to about 0.7 volt and theinternal feedback mechanism within the equivalent transistor structuresof the main thyristor interrupted. The high impedance blocking state isthus reestablished in main thyristor 23 while the load voltage stillremains applied to the device. In this connection, it is seen that thecurrent gain of the NPN transistor formed by diode 34 and commonanode-base region 26 at diode 34 must be low to provide for effectivegate turn-off. Otherwise, the negative gate signal would cause injectionacross the common cathode-base region 25 and turn-off of the mainthyristor.

Referring to FIGS. 4 and 5, an alternative embodiment of the integratedamplifying gate thyristor with gate assist turn-off capability is shown.The structure is the same as that described in connection with FIGS. 2and 3 except that the annular segments of cathode-emitter region 132 ofpilot thyristor 131 and cathode region 135 of diode 134 are radiallyoffset and in turn gate electrode 139 and floating electrode 140 arechanged as shown. Accordingly, the elements and features arecorrespondingly numbered to the embodiment shown in FIGS. 2 and 3, withthe prefix "1" before each number.

This embodiment is preferred because it enables the current gain of theNPN transistor structure to be more readily controlled by meanscomplimentary to irradiation as hereinbefore described. Irradiation mask150 can thus be made with openings 151 forming a complete annulus exceptfor support portions 153, and mask 150 is more easily aligned over theannular segments of diode 134. In this connection, it should be notedthat cathode and gate electrodes 138 and 139 are shaped so that theannular segments of pilot thyristor 131 are positioned centrally of theannular segments of diode 134. This relative positioning is importantbecause the pilot thyristor triggers along its inner edge toward gateelectrode 139 and the diode injects electrons into common cathode-baseregion 125 along its outer edge on application of the negativegate-assist turn-off current.

The operation of the integrated amplifying gate thyristor with gateassist turn-off capability shown in FIGS. 4 and 5 is the same as thatshown and described in connection with FIGS. 2 and 3 (as shown by thearrows).

Referring to FIGS. 6, 7 and 8, another integrated amplifying gatethyristor with gate assist turn-off capability of the present inventionis shown. All parts are the same as the thyristor described inconnection with FIGS. 2 and 3 except for cathode region 235 of diode234, which is provided with shallow impurity lips 250 toward mainthyristor 223. The operation is fundamentally the same as that describedin connection with FIGS. 2 and 3. Accordingly, the parts arecorrespondingly numbered to the elements and features of the thyristordescribed in connection with FIGS. 2 and 3, with the prefix "2" in frontof each number.

Shallow impurity lips 250 are at least as large in area and preferablygreater than about twice the area of the deep portions 251 of cathoderegion 235, and are at least one-half and preferably less than aboutone-third the depth of the deep portions 251 of cathode region 235.Shallow impurity lips 250 are preferably formed by standard diffusiontechniques along with cathode-emitter region 224, cathode-emitter region232, and deep portions 251 of cathode region 235. The deep diffusion ispreferably interrupted to remove additional portions of the diffusionmask corresponding to the areas of first major surface 221 through whichlips 250 are to be diffused. This removal of the additional portions ofthe diffusion mask is accomplished by standard photolithographic andetch techniques. The shallow lips 250 are then diffused through thenewly exposed portions of major surface 221, while the diffusion of thedeep impurity regions 224, 232 and 235 is completed.

Shallow impurity lips 250 substantially lower the current gain of theNPN transistor structure formed by cathode region 235 of diode 234 andcommon cathode-base and anode-base regions 235 and 236 at diode 234 bycausing electron injection into common cathode-base region 225 to occurin lips 250. With shallow lips 250, electron injection will initiate atthe outer edge of deep portion 251 and further electron injection willoccur progressively over the area of shallow lips 250. Electroninjection through the diode is thus forced to occur over a larger areain which the common cathode-base region 225 is wider. In turn, theinjection efficiency for electrons into the cathode-base region 225 andthe transport factor for electrons through the cathode-base region 225are reduced. All of this substantially lowers the current gain of theNPN transistor structure at diode 234. The current gain can be furtherreduced, if need be, by selectively irradiating the diode (includinglips 250) as above described in connection with FIGS. 2 and 3 toselectively increase the recombination rate in the common cathode-baseregion 225 at diode 234.

The use of shallow impurity lips 250 also assists in the efficiency ofturn-on of the device by enabling cathode regions 235 of diode 234 to bemore deeply diffused without effecting the current gain of the NPNstructure at the diode. The lateral resistance through cathode-baseregion 225 at deep portions 251 of diode 234 can in turn be increased.The lateral current through cathode-base region 225 on turn-on can thusbe diverted to flow more under cathode-emitter region 232 of pilotthyristor 231 -- increasing the efficiency of the gate turn-on current.The amount of wasted gate current by-passing the active element (i.e.pilot thyristor or diode) is reduced in both turn-on and turn-off.

While the presently preferred embodiments of the invention and methodsof making them have been specifically described, it is distinctlyunderstood that the invention may be otherwise variously embodied andused within the scope of the following claims. For example, it isreadily apparent from the embodiments shown that the pilot thyristor andthe diode can be arranged in various configurations depending uponwhether turn-off capability or turn-on capability is emphasized.Furthermore, although the N-type and P-type regions are preferablyformed by diffusion, epitaxial growth or a combination of epitaxialgrowth and diffusion, may be utilized in forming the various impurityregions in the semiconductor body. In addition, although it iscontemplated that silicon will typically be utilized for thesemiconductor body, that other semiconductor materials such as germaniumand gallium arsenide may be utilized to achieve specified electricalcharacteristics for a particular device.

What is claimed is:
 1. A method of making an integrated amplifying gatethyristor with gate assist turn-off capability comprising the stepsof:A. providing in a semiconductor body having first and second opposedmajor surfaces impurities to form in the body a main thyristor of fourimpurity regions of alternate type conductivity disposed alternatelythrough the body between the major surfaces with a PN junction betweeneach region, with one intermediate region adjoining the first majorsurface, the two impurity regions adjoining the first and second majorsurfaces being cathode-emitter and anode-emitter regions, respectively,the two intermediate impurity regions adjoining the cathode-emitter andanode-emitter regions being cathode-base and anode-base regions,respectively, and the cathode-base region adjoining the first majorsurface adjacent the cathode-emitter region; B. providing in thesemiconductor body an impurity to form in the body a pilot thyristor ofsaid cathode-base, anode-base and anode-emitter regions of the mainthyristor and at least one additional impurity region adjacent to andspaced from the cathode-emitter region of the main thyristor ofconductivity-type to form a PN junction with said cathode-base regionand a separate cathode-emitter region for the pilot thyristor; C.providing in the semiconductor body an impurity to form in the body adiode of said cathode-base region of the thyristors and at least oneadditional impurity region being a cathode region adjacent to and spacedfrom the cathode-emitter regions of the main and pilot thyristors; D.selectively masking against a radiation means portions of the firstmajor surface of the semiconductor body adjoining the main and pilotthyristor, while leaving exposed portions of the first major surfaceadjoining the diode; E. selectively irradiating the diode in thesemiconductor body with said radiation means through said masking toreduce the current gain of the common cathode-base region at the diodeto less than the ratio of I_(FB) /I_(g), where I_(FB) is the forwardanode current triggering the diode to function as a thyristor in the lowimpedance conducting state, and I_(g) is a specified gate assistturn-off current to turn-off the main thyristor; F. positioning cathodeand anode electrodes on said first and second major surfaces,respectively, to make ohmic contact with the cathode-emitter region ofthe main thyristor and the common anode-emitter region of the main andpilot thyristors; G. positioning a gate electrode on said first majorsurface to make ohmic contact with the common cathode-base regionadjacent to and spaced from the cathode-emitter region of the pilotthyristor and the cathode region of the diode; and H. positioning afloating electrode on said first major surface adjacent to and spacedfrom the gate electrode to make ohmic contact with the commoncathode-base region between the thyristors and the cathode-emitterregion of the pilot thyristor.
 2. A method of making an integratedamplifying gate thyristor with gate assist turn-off capability as setforth in claim 1 wherein:the radiation source is an electron beam.
 3. Amethod of making an integrated amplifying gate thyristor with gateassist turn-off capability as set forth in claim 2 wherein:theirradiation of the diode in the semiconductor body with the electronbeam is to a dosage level greater than about 1 × 10¹³ electrons/cm². 4.A method of making an integrated amplifying gate thyristor with gateassist turn-off capability as set forth in claim 3 wherein:the electronbeam has an energy greater than about 1 Mev.
 5. A method of making anintegrated amplifying gate thyristor with gate assist turn-offcapability as set forth in claim 3 wherein:said cathode region of thediode is provided with a shallow impurity lip extending toward thecathode-emitter region of the main thyristor.